About 4 million people in the U.S. seek medical care each year for shoulder problems, with shoulder arthritis as one of the most common. Shoulder replacement surgery, also called total shoulder arthroplasty or "TSA", is often the only effective treatment for the severe, unrelenting pain that affects these patients. TSA is a difficult surgical procedure, which requires accurate placement of prostheses through a small, deep incision, which offers very little visibility of the affected anatomy. Replacement of the glenoid surface is the most difficult part of the procedure, and misalignment of the glenoid implant can lead to implant loosening, accelerated implant wear, and eventual revision surgery. The Overall Aim of this project is to develop a complete computer assisted surgical planning and navigation system for total shoulder arthroplasty. This will allow a surgeon to execute a patient-optimized plan more quickly and more accurately with fewer risks and costs. Phase I Hypothesis: The combination of image-based planning and surgical navigation will improve the accuracy of glenoid placement. Specific Aim 1 - Develop proof of concept preoperative planner for TSA allowing implant sizing and orientation against the patient's preoperative CT data and simulation based upon a simplified joint model. Specific Aim 2 - Develop electromagnetic tracking-compatible tooling for TSA navigation in order to replace bulkier optical trackers that are typically used in computer-assisted joint replacement. Specific Aim 3 - Develop a proof of concept intraoperative TSA navigation system to aid in glenoid surface preparation and implant placement, including incorporation of optimal bone registration parameters. Specific Aim 4: Show the feasibility and benefits of TSA planning/navigation for glenoid placement by comparing the new system against standard surgical practice in a series of mock surgeries on foam bone models. The proposed work will validate computer-assisted techniques in total shoulder replacement. The presented technology will allow surgeons to accurately plan and execute implantation of the prosthetic shoulder socket, thereby improving effectiveness of the prosthesis and lowering the risk of postoperative complications. [unreadable] [unreadable] [unreadable] [unreadable]